1. Field of the Invention
This invention concerns an air/fuel ratio control system for purging the evaporated fuel generated in the fuel tank of an internal combustion engine and discharging the evaporated fuel to the intake side of the engine.
2. Description of the Related Art
Japanese Patent Application Kokai No. 5-187332 discloses a system for an internal combustion engine which learns the drive start signal of the engine's flow volume control valve or purge control valve during idle to carry out linear correction of the flow volume characteristics of the flow volume control valve, thereby preventing deterioration of exhaust emission quality. Japanese Patent Application Kokai No. 5-215020 discloses sequential detection of the flow volume in the flow volume control valve with a flowmeter to correct the drive signal and accurately control the purge volume of the flow volume control valve.
In both of these cases, when the engine is not idling, if the difference of the former and latter pressure of the flow volume control valve should change, the duty ratio flow volume characteristics of that flow volume control valve change, so that it is impossible to completely prevent a worsening of exhaust emission quality with simple linear correction procedures. Also, the nominal error of the flow volume control valve, i.e., differences in relation to the design logic values which arise from differences in the unique part production dimensions of individual flow volume control valves occurs even with a high duty ratio (about 90%) so that it is not possible to make entirely accurate corrections. Furthermore, the latter also requires a flowmeter, making the structure more complex.
In addition, regarding such an air-fuel ratio control device, if phenomena occur in the process of learning and implementing control of the valve, such as that the learning values extend to the upper and lower values of the learning routine or that the air/fuel ratio sensor output becomes unstable, stabilization of the air/fuel ratio is delayed and it is impossible to carry out the evaporation purge process until the air/fuel ratio stabilizes. If the evaporation purge process is stopped in this way for an excessively long period, the evaporation purge adsorption volume of the canister will attain a saturated state, thereby making any further adsorption impossible.
Moreover, with such an air/fuel ratio control device the fuel injection amount of the injector is corrected in relation to the recommended density value of the evaporated gas (hereafter referred to as the evaporated gas density). As a result, in order to realize accurate air/fuel ratio control in relation to the evaporation purge-process, it is important to accurately assess the evaporated gas density. A variety of methods have already been proposed for assessing the evaporated gas density. See, for example, Japanese Patent Application Kokai No. 5-288107.
However, there is a further problem with conventional air/fuel ratio control devices. Carrying out air/fuel ratio control with the estimated evaporated gas density produces a rough value so that if the evaporated gas density suddenly changes, it takes time for the evaporated gas density, i.e., the estimated value, to agree with the actual density. In particular, because the evaporated gas density increases during engine start and fueling, this factor can easily lead to discrepancies in the air/fuel ratio.